Sounds like diverter valve on boiler, dont quote me on that though. On a combi the diverter valve can sieze up cos it made up of a brass valve with a diapraghm(large rubber washer) inside and these tend to become brittle and sieze up. The preset postion its in is for hot water, so thats why your hot water works. Combis are a nightmare, I just fit them, dont repair them yet so it could possibly be something else.